1. Field of the Invention
This invention relates to a method and system for synchronizing the movement of plural recording media and, more particularly, to such a method and system advantageously applicable to controlling the speeds of recording tape in video tape recorders so that information can be transferred therebetween at proper frames on the recording tapes.
2. Description of the Prior Art
The technique of synchronizing plural recording media, such as the recording tapes of two video tape recorders (VTR's) is known, as described in U.S. Pat. No. 3,911,488. According to this technique, an up/down counter is responsive to frame signals which are reproduced from the respective tapes. Frame signals which are reproduced from one tape increment the count of the counter, while frame signals which are reproduced from the other tape decrement the count. Initially, the counter is preset to some intermediate count, and this preset count either is increased or decreased depending upon whether the speed of one tape is greater or less than the speed of the other tape. A digital-to-analog converter is responsive to the instantaneous count of the counter and provides an analog rate control signal having an amplitude and polarity which simultaneously increases one tape speed while retarding the other. It is assumed that proper synchronism is attained when the counter resumes its intermediate count for a sufficient period of time, whereupon the rate control signals are disabled and a standard reference signal is used to control the speed of the tape.
The up/down counter which is used in this patent is preset to its intermediate count when a selected frame associated with one tape is reached. As this tape is further advanced, the count is increased. When the desired frame of the second tape is reached, the frame pulses which are reproduced from that second tape are supplied to the counter to reduce the count thereof. Further operation of the counter is dependent solely upon the frame signals, and there is no actual comparison between the instantaneous address reproduced from the first tape with the instantaneous address reproduced from the second tape. If this technique is used to control an editing operation between two VTR's, then it is necessary that the first tape be rewound from a selected frame by a certain amount, and that the second tape be rewound from its selected frame by at least that same amount. If one or the other tape is rewound by less than the required amount, then it is possible that those particular frames which are used to preset the counter and then to decrement the counter may never be reached. Consequently, a relatively long delay must be provided in order to assure that the tapes will be advanced in synchronism. The tapes cannot be rewound merely by an arbitrary amount; rather, it is necessary to rewind at least preset lengths of tape.
In U.S. Pat. No. 3,851,116, the speed of a single magnetic tape is controlled as a function of the distance between the actual position of the tape and a desired position therealong. The tape is divided into successive frames and the address of each frame is decoded and compared to a preselected address. The tape speed is variable according to the difference between the actual and predetermined addresses. However, this patent is limited solely to speed control over a single tape. There is, of course, no recognition of the problem of synchronizing two different tapes to run at precisely the same speed when selected addresses of the tapes are reached.
An analogous film speed control technique is described in U.S. Pat. No. 4,066,349 wherein a film strip is driven until a selected film frame is reached. Assuming that the actual frame being scanned differs from the desired frame, a ramp generator is enabled to produce a gradually increasing signal. This signal serves to increase the film speed. Once the film speed exceeds a predetermined level, a higher speed motor is energized to drive the film at a higher fixed speed. This advances the film beyond the desired frame, resulting in a reversed operation wherein the film then is driven in the opposite direction, first in response to the ramp signal and then in response to the high speed motor. This "hunting" about the desired frame continues until that frame ultimately is reached. Not only is this technique not suitable for driving two different film strips, or tapes, in synchronism, but the aforementioned hunting operation is time consuming and not very accurate.